The present invention relates generally to electrical instrumentation and more particularly to the monitoring of the intensity and position of electrically charged beam bunches within transport systems.
The present invention was devised in response to the need for monitoring the intensity and position of beam bunches of charged particles in a synchrotron radiation facility having a linear accelerator, a storage ring, and an interconnecting transport line. In the particular facility involved, an electron gun having a very short (0.7 ns) rise time is used to provide two modes of operation. In one mode, beam bunches of 4 ns are produced for injection purposes, while in the other mode, much longer beam bunches (in the order of 2 .mu.s) are produced for nuclear physics experiments.
Critically aligned experiments can be very sensitive to very small changes in the trajectory of the beam. In some cases, a 50 micron shift, or less, of the beam's intensity maximum at a narrow entrance slit can cause a significant loss in intensity and an increase of beam noise, particularly at high photon energies. These stringent alignment tolerances necessitate a steering control system capable of making and maintaining fine corrections to the position of the beam bunches within the lines of the synchrotron system.
A fundamental component of such a steering system is the instrumentalism used to sense the intensity and position of the beam bunches within the synchrotron line during the different modes of operation. The accuracy and stability of steering adjustments are intrinsically limited by the performance characteristics of the monitors. With the tighter alignment tolerances of present systems, a relatively large number of beam monitors is needed.
What is desired is the monitoring instrumentation is a monitor device which can be used at a desired location in the beam path to provide high resolution measurements of the intensity of the beam and its position relative to the axis of the transport line, for both modes of operation, and with output signals which are of similar shape so that they can be handled by a single signal processing system.
Prior to the present invention, no simple, single monitor device existed which fulfilled this need.
Gap monitors have been used to measure the total intensity of beam bunches. Basically, these moniors consists of a circumferential gap in the beam pipe through which the beam bunches are passing, with a voltage being developed across the gap as the beam bunches pass thereby. In some instances, the gap is filled with a ceramic material to insulate between the ends of the beam pipe and maintain the vacuum within the beam pipe, with the output signal being developed across a resistor connected between the ends of the beam pipe. In other gap monitors a coaxial structure is provided to serve as a pressure tight gap enclosure. In such case, it is customary to stack ferrites around the beam pipe and in the gap enclosure. For beam bunches of a length much longer then the gap the ferrites serve to load the gap inductively and thereby improve the low frequency response. For narrow bunches, wherein the bunch length is comparable to the gap, the ferrites serve an entirely different purpose and absorb the reflections in the gap enclosure. The beam pipe is circumferentially electrically continuous at each side of the gap and thus the gap signal, which is proportional to the total intensity of the beam, does not provide information as to the location of the beam relative to the axis of the beam pipe.
Stripline monitors have been developed to provide output signals which will enable the position of the beam bunches relative to the beam pipe axis to be determined. Typically a stripline monitor consists of four conducting strips extending lengthwise of a section of beam pipe, the strips being symmetrically located relative to the beam pipe axis and spaced inwardly of the inner beam pipe wall. The physical relationship of the width of the strips, the diameter of the beam pipe and the spacing from the strips to the beam pipe wall will determine the characteristic impedance of the strips. The strips are connected at one end to the beam pipe, either by shorting them to the beam pipe or by terminating each strip through a resistance equal to the characteristic impedance, with the other ends of the strips being used as outputs.
In general, as a beam bunch passes through the beam pipe, output signals will be induced in each of the four striplines. The magnitude of the signal in each strip is proportional to the beam intensity, or current, and inversely proportional to the distance between the beam path and the strip. The sum and difference in signals from opposite strips can thus be used to measure the location of the beam relative to the axis of the beam pipe.
The signals from the form striplines can also be summed to provide a measurement of total beam intensity. However, in order to provide for proper impedance of the striplines and good sensitivity for position measurement, the strips should be relatively narrow. As a consequence, they can only pick up a small part of the wall current signal induced by the beam, with a consequent sacrifice in precision of a total intensity measurement.